1. Technical Field
The present invention relates to ambient light photosensors and to a display device using such. More particularly it relates to a display device with improved ambient light photosensor directionality.
2. Related Art
Over recent years the use of display devices has spread rapidly not only in telecommunications equipment but also in electrical equipment in general. To reduce power consumption, ambient light photosensors have been installed in liquid crystal panels, especially in those for portable equipment. The method used is that the ambient light photosensors sense the brightness of the ambient light, and the backlight or similar is controlled to turn on/off according to the results of such sensing.
An example which will serve to describe the operation of such an ambient light photosensor is the ambient light photosensor set forth in JP-A-2001-169190, wherein thin film transistors (TFTs) for photosensing are disposed on the substrate of a liquid crystal display panel, and the device has the ability, through sensing of the TFTs' light leakage current, to have the backlight turn on/off automatically according to the brightness of the surroundings. It is set forth therein how introduction of such ambient light photosensors reduces wasteful power consumption and renders the display screen better viewable through having brightness that matches the ambient conditions.
Ambient light photosensors of the related art are disposed in the display area's vacant regions where they will receive ambient light, as for example shown in FIG. 9. Electrical current flows in the semiconductor layer of the ambient light photosensors as a result of ambient light striking the photosensing portion thereof. When such current flows, the accumulated electrical charge is discharged. The duration of such discharge is proportional to the intensity of the ambient light, the photosensors having the characteristic that the more intense the ambient light, the faster the speed at which the charge is discharged. Methods have also been examined whereby, to match such purpose, multiple ambient light photosensors are disposed around the display area's periphery so as to increase the area over which light shines onto the light sensors and render the light more readily absorbed, thereby raising the ambient light photosensor sensitivity. Also, if a single ambient light photosensor is disposed in a vacant region of some equipment incorporated in the display device, there will be situations where the photosensor is blinded by some inadvertent movement of the user's fingers, or where, if light alternately shines on and is blocked from the photosensor for momentary durations due to the effects of the surrounding environment, the brightness of the backlight will frequently fluctuate or be irregular in other ways and it will not necessarily be possible to maintain the backlight at the desired brightness. The method of disposing multiple ambient light photosensors at the display periphery was also devised in order to avoid situations of that kind.
Accordingly, methods were devised whereby ambient light photosensors are arrayed at the periphery of the display area, in directions parallel to the signal lines and parallel to the scan lines, as for instance set forth in JP-A-9-34635. This was in order, by designating the ambient light photosensors parallel to the signal lines as “Y” and those parallel to the scan lines as “X”, and by expressing the intersections of two light-receiving elements as (X, Y) and causing shade to appear at such intersections, to use the photosensors as a pointing device for specifying X, Y coordinate two-dimensional positions.
Following that, a method was devised whereby the external wiring and ambient light photosensors are disposed with the display unit therebetween, and there are built-in image sensors on at least one of the display unit's edges, parallel to such edge(s), as set forth in JP-A-11-84426. JP-A-11-84426 describes a structure whereby n ambient light photosensors are disposed in locations distanced from the terminals of the leads to the external wiring, and from the display periphery circuits, in order to prevent the ambient light photosensor sensitivity from falling due to the effects of heat emitted by the display periphery circuits or noise from the high frequency signals. Thereby, ambient light photosensors with good sensitivity are provided.
Ambient light photosensors of the related art are, as FIG. 9 shows, disposed with the drive circuits and display area therebetween, on opposite sides or in directions parallel to the signal lines. However, ambient light photosensors are disposed parallel to the signal lines only when the wiring is one-sided, as in other cases they would be obstructed by the external wiring. But with one-sided wiring, regions where the ambient light photosensors can be disposed exist only on one side, so that there are restrictions on the locations where the photosensors can be disposed. Also, the spacing between the scan lines is narrow, and when the ambient light photosensors are disposed, the size of one ambient light photosensor will be that of one such spacing between the scan lines, as shown in FIG. 10Z. As a result, the sensitivity of the ambient light photosensors will not be high, and it will be difficult to accommodate the light sensor layout to the differing device case specifications of each user. Accordingly, there has arisen a need to raise the directionality, and to raise the light sensor sensitivity, by diversifying the light sensor disposition locations beyond the layouts of the related art. As mentioned in the foregoing descriptions of the related art, JP-A-9-34635 and JP-A-11-84426 disclose structures which have a display unit equipped with sensor units, plus peripheral drive circuit portions, and in which the peripheral drive circuit portions are disposed along at least one edge of the display unit, and image sensors are disposed along another edge of the display unit. However, JP-A-9-34635 sets forth a pointing device, and the basic structure therein is at variance with the intended purpose of sensors. The photosensors set forth therein are characterized by being disposed in longitudinal and lateral directions at the display area periphery, and function as pointing sensors enabling determination of the positions of the intersections (X, Y) of those two directions. Therefore, the photosensors will have lengths that are the same as the lengths of the side edges of the display area, and the lengths of the light sensors parallel to the signal lines are likely to differ from those parallel to the scan lines, depending on the panel's inch-size and shape. Further, in JP-A-11-84426, it is set forth that the locations where the light sensors are disposed are distanced from the peripheral drive circuit portions, the light sensors being disposed at display area peripheral portions that have the peripheral drive circuit portions located therebetween. Also, the light sensors set forth in JP-A-11-84426 may be disposed in various different sizes, and thus the structure is not one that pays heed to photo-directionality. Further, no consideration is given therein to the wiring regions occupied by pixel drive wiring, and thus it would be difficult to dispose multiple sensors close to the display area.
Where ambient light photosensors parallel to the signal lines and ambient light photosensors parallel to the scan lines are provided, a structure is anticipated in which either one or the other of such sensor sets will be employed, according to the user's specification. If the size of the ambient light photosensors disposed parallel to the signal lines differs from that of the ambient light photosensors disposed parallel to the scan lines, then the photo-directionality and photo-sensitivity of such two sensor sets will differ, and consequently situations will arise in which the sensitivity in one direction is good but the sensitivity in the other direction is poor. And there is also concern that, not being disposed in positions proximate to the display area periphery, the light sensors will be hidden by the device's case and be unable to fulfill their function.